gcc/tree-ssa-loop-ch.c

   1 /* Loop header copying on trees.
   2    Copyright (C) 2004-2014 Free Software Foundation, Inc.
   3
   4 This file is part of GCC.
   5
   6 GCC is free software; you can redistribute it and/or modify it
   7 under the terms of the GNU General Public License as published by the
   8 Free Software Foundation; either version 3, or (at your option) any
   9 later version.
  10
  11 GCC is distributed in the hope that it will be useful, but WITHOUT
  12 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14 for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GCC; see the file COPYING3.  If not see
  18 <http://www.gnu.org/licenses/>.  */
  19
  20 #include "config.h"
  21 #include "system.h"
  22 #include "coretypes.h"
  23 #include "tm.h"
  24 #include "tree.h"
  25 #include "tm_p.h"
  26 #include "basic-block.h"
  27 #include "tree-ssa-alias.h"
  28 #include "internal-fn.h"
  29 #include "gimple-expr.h"
  30 #include "is-a.h"
  31 #include "gimple.h"
  32 #include "gimple-iterator.h"
  33 #include "gimple-ssa.h"
  34 #include "tree-cfg.h"
  35 #include "tree-into-ssa.h"
  36 #include "tree-pass.h"
  37 #include "cfgloop.h"
  38 #include "tree-inline.h"
  39 #include "flags.h"
  40 #include "tree-ssa-threadedge.h"
  41
  42 /* Duplicates headers of loops if they are small enough, so that the statements
  43    in the loop body are always executed when the loop is entered.  This
  44    increases effectiveness of code motion optimizations, and reduces the need
  45    for loop preconditioning.  */
  46
  47 /* Check whether we should duplicate HEADER of LOOP.  At most *LIMIT
  48    instructions should be duplicated, limit is decreased by the actual
  49    amount.  */
  50
  51 static bool
  52 should_duplicate_loop_header_p (basic_block header, struct loop *loop,
  53                                 int *limit)
  54 {
  55   gimple_stmt_iterator bsi;
  56   gimple last;
  57
  58   /* Do not copy one block more than once (we do not really want to do
  59      loop peeling here).  */
  60   if (header->aux)
  61     return false;
  62
  63   /* Loop header copying usually increases size of the code.  This used not to
  64      be true, since quite often it is possible to verify that the condition is
  65      satisfied in the first iteration and therefore to eliminate it.  Jump
  66      threading handles these cases now.  */
  67   if (optimize_loop_for_size_p (loop))
  68     return false;
  69
  70   gcc_assert (EDGE_COUNT (header->succs) > 0);
  71   if (single_succ_p (header))
  72     return false;
  73   if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest)
  74       && flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 1)->dest))
  75     return false;
  76
  77   /* If this is not the original loop header, we want it to have just
  78      one predecessor in order to match the && pattern.  */
  79   if (header != loop->header && !single_pred_p (header))
  80     return false;
  81
  82   last = last_stmt (header);
  83   if (gimple_code (last) != GIMPLE_COND)
  84     return false;
  85
  86   /* Approximately copy the conditions that used to be used in jump.c --
  87      at most 20 insns and no calls.  */
  88   for (bsi = gsi_start_bb (header); !gsi_end_p (bsi); gsi_next (&bsi))
  89     {
  90       last = gsi_stmt (bsi);
  91
  92       if (gimple_code (last) == GIMPLE_LABEL)
  93         continue;
  94
  95       if (is_gimple_debug (last))
  96         continue;
  97
  98       if (is_gimple_call (last))
  99         return false;
 100
 101       *limit -= estimate_num_insns (last, &eni_size_weights);
 102       if (*limit < 0)
 103         return false;
 104     }
 105
 106   return true;
 107 }
 108
 109 /* Checks whether LOOP is a do-while style loop.  */
 110
 111 static bool
 112 do_while_loop_p (struct loop *loop)
 113 {
 114   gimple stmt = last_stmt (loop->latch);
 115
 116   /* If the latch of the loop is not empty, it is not a do-while loop.  */
 117   if (stmt
 118       && gimple_code (stmt) != GIMPLE_LABEL)
 119     return false;
 120
 121   /* If the header contains just a condition, it is not a do-while loop.  */
 122   stmt = last_and_only_stmt (loop->header);
 123   if (stmt
 124       && gimple_code (stmt) == GIMPLE_COND)
 125     return false;
 126
 127   return true;
 128 }
 129
 130 /* For all loops, copy the condition at the end of the loop body in front
 131    of the loop.  This is beneficial since it increases efficiency of
 132    code motion optimizations.  It also saves one jump on entry to the loop.  */
 133
 134 namespace {
 135
 136 const pass_data pass_data_ch =
 137 {
 138   GIMPLE_PASS, /* type */
 139   "ch", /* name */
 140   OPTGROUP_LOOP, /* optinfo_flags */
 141   TV_TREE_CH, /* tv_id */
 142   ( PROP_cfg | PROP_ssa ), /* properties_required */
 143   0, /* properties_provided */
 144   0, /* properties_destroyed */
 145   0, /* todo_flags_start */
 146   TODO_cleanup_cfg, /* todo_flags_finish */
 147 };
 148
 149 class pass_ch : public gimple_opt_pass
 150 {
 151 public:
 152   pass_ch (gcc::context *ctxt)
 153     : gimple_opt_pass (pass_data_ch, ctxt)
 154   {}
 155
 156   /* opt_pass methods: */
 157   virtual bool gate (function *) { return flag_tree_ch != 0; }
 158   virtual unsigned int execute (function *);
 159
 160 }; // class pass_ch
 161
 162 unsigned int
 163 pass_ch::execute (function *fun)
 164 {
 165   struct loop *loop;
 166   basic_block header;
 167   edge exit, entry;
 168   basic_block *bbs, *copied_bbs;
 169   unsigned n_bbs;
 170   unsigned bbs_size;
 171
 172   loop_optimizer_init (LOOPS_HAVE_PREHEADERS
 173                        | LOOPS_HAVE_SIMPLE_LATCHES);
 174   if (number_of_loops (fun) <= 1)
 175     {
 176       loop_optimizer_finalize ();
 177       return 0;
 178     }
 179
 180   bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
 181   copied_bbs = XNEWVEC (basic_block, n_basic_blocks_for_fn (fun));
 182   bbs_size = n_basic_blocks_for_fn (fun);
 183
 184   FOR_EACH_LOOP (loop, 0)
 185     {
 186       /* Copy at most 20 insns.  */
 187       int limit = 20;
 188
 189       header = loop->header;
 190
 191       /* If the loop is already a do-while style one (either because it was
 192          written as such, or because jump threading transformed it into one),
 193          we might be in fact peeling the first iteration of the loop.  This
 194          in general is not a good idea.  */
 195       if (do_while_loop_p (loop))
 196         continue;
 197
 198       /* Iterate the header copying up to limit; this takes care of the cases
 199          like while (a && b) {...}, where we want to have both of the conditions
 200          copied.  TODO -- handle while (a || b) - like cases, by not requiring
 201          the header to have just a single successor and copying up to
 202          postdominator.  */
 203
 204       exit = NULL;
 205       n_bbs = 0;
 206       while (should_duplicate_loop_header_p (header, loop, &limit))
 207         {
 208           /* Find a successor of header that is inside a loop; i.e. the new
 209              header after the condition is copied.  */
 210           if (flow_bb_inside_loop_p (loop, EDGE_SUCC (header, 0)->dest))
 211             exit = EDGE_SUCC (header, 0);
 212           else
 213             exit = EDGE_SUCC (header, 1);
 214           bbs[n_bbs++] = header;
 215           gcc_assert (bbs_size > n_bbs);
 216           header = exit->dest;
 217         }
 218
 219       if (!exit)
 220         continue;
 221
 222       if (dump_file && (dump_flags & TDF_DETAILS))
 223         fprintf (dump_file,
 224                  "Duplicating header of the loop %d up to edge %d->%d.\n",
 225                  loop->num, exit->src->index, exit->dest->index);
 226
 227       /* Ensure that the header will have just the latch as a predecessor
 228          inside the loop.  */
 229       if (!single_pred_p (exit->dest))
 230         exit = single_pred_edge (split_edge (exit));
 231
 232       entry = loop_preheader_edge (loop);
 233
 234       propagate_threaded_block_debug_into (exit->dest, entry->dest);
 235       if (!gimple_duplicate_sese_region (entry, exit, bbs, n_bbs, copied_bbs,
 236                                          true))
 237         {
 238           fprintf (dump_file, "Duplication failed.\n");
 239           continue;
 240         }
 241
 242       /* If the loop has the form "for (i = j; i < j + 10; i++)" then
 243          this copying can introduce a case where we rely on undefined
 244          signed overflow to eliminate the preheader condition, because
 245          we assume that "j < j + 10" is true.  We don't want to warn
 246          about that case for -Wstrict-overflow, because in general we
 247          don't warn about overflow involving loops.  Prevent the
 248          warning by setting the no_warning flag in the condition.  */
 249       if (warn_strict_overflow > 0)
 250         {
 251           unsigned int i;
 252
 253           for (i = 0; i < n_bbs; ++i)
 254             {
 255               gimple_stmt_iterator bsi;
 256
 257               for (bsi = gsi_start_bb (copied_bbs[i]);
 258                    !gsi_end_p (bsi);
 259                    gsi_next (&bsi))
 260                 {
 261                   gimple stmt = gsi_stmt (bsi);
 262                   if (gimple_code (stmt) == GIMPLE_COND)
 263                     gimple_set_no_warning (stmt, true);
 264                   else if (is_gimple_assign (stmt))
 265                     {
 266                       enum tree_code rhs_code = gimple_assign_rhs_code (stmt);
 267                       if (TREE_CODE_CLASS (rhs_code) == tcc_comparison)
 268                         gimple_set_no_warning (stmt, true);
 269                     }
 270                 }
 271             }
 272         }
 273
 274       /* Ensure that the latch and the preheader is simple (we know that they
 275          are not now, since there was the loop exit condition.  */
 276       split_edge (loop_preheader_edge (loop));
 277       split_edge (loop_latch_edge (loop));
 278     }
 279
 280   update_ssa (TODO_update_ssa);
 281   free (bbs);
 282   free (copied_bbs);
 283
 284   loop_optimizer_finalize ();
 285   return 0;
 286 }
 287
 288 } // anon namespace
 289
 290 gimple_opt_pass *
 291 make_pass_ch (gcc::context *ctxt)
 292 {
 293   return new pass_ch (ctxt);
 294 }